Compounds of the general formula CF.sub.2 .dbd.CF--R.sub.f --CN wherein R.sub.f is perfluoroalkyl or perfluoroalkoxy containing 2-20 carbon atoms are useful as cure site monomers in fluoroelastomers. See, for example, U.S. Pat. No. 3,546,186 (Gladding & Sullivan) or U.S. Pat. No. 4,281,092 (Breazeale). A traditional method for making such compounds is by (a) first reacting the corresponding ester starting material (or a derivative thereof) with ammonia gas at low cooling temperature to form an amide compound, and (b) then converting the amide to the desired nitrile through a dehydration process.
The traditional synthetic approach suffers from low yield due to the problems of a non-homogeneous reaction medium and from difficulties in product separation. Since the solubilities of fluorinated compounds are usually quite low in common organic solvents, a fluorinated solvent such as 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) is often used in these reactions as a product extraction solvent. CFCs are now undesirable for environmental reasons, and their manufacture is being phased out. Formation of significant quantities of side-products due to sensitivity of olefin functionality toward the reaction reagents also occurs. See U.S. Pat. No. 4,138,426 (England). The yield from the first step of England Example 8, conducted in ether (ethyl ether), was only 9%, and CFC-113 was employed in separating the reaction product from the reaction mass. In the second step of England, the first step amide was dehydrated to the nitrile in tetrahydrofuran solvent using pyridine and trifluoroacetic anhydride as dehydrating agent. Again, CFC-113 was used to aid separation of the product from the reaction mass.
An improved method of synthesizing these compounds is desired.